To date there have been numerous proposals for the design of read-head sensors in high density magnetic recording. The proposals are included in U.S. Pat. No. 5,155,643 by R. E. Jones Jr. et al., issued Oct. 13, 1992, entitled "Unshielded Horizontal Magnetoresistive Head and Method of Fabricating Same"; pending U.S. patent application Ser. No. 08/688,583 entitled "A Self-Biasing, Non-Magnetic, Giant Magnetoresistance Sensor"; pending U.S. patent application Ser. No. 08/435,254, entitled "Process for Forming a Magnetoresistive Sensor for a Reading Head"; an article by J. R. Childress et al. entitled "New Epitaxial Multilayer System for Spin-Valve Magnetic Sensors" in Applied Physics Letters, Vol. 63, 1996 (1993); U.S. Pat. No. 5,166,849, by J. M. Fedeli, issued Nov. 24, 1992, entitled "Horizontal Magnetic Head With Hall Effect and its Embodiment Methods"; and an article by R. S. Indeck et al, entitled "A Magnetoresistive Gradiometer" in IEEE Trans. Mag., Vol. 24, 2617 (1988). The latter four proposals all involve the use of detection elements which lack vertical spatial resolution in the direction normal to the plane of the magnetic track. But the strength of the magnetic field constituting the signal falls off extremely rapidly with distance above the surface of the recording medium. Therefore, higher sensitivity is achievable by selectively sampling the signal field only in the region very close to the surface of the recording medium. In practice this requires not only a very small flying height less than 1000 .ANG. but also a "thin" detector with a thickness in the vertical direction of less than 500 .ANG.. A further advantage of a thin detector is that it needs no shielding since it senses only the local field at the surface of the recording medium as described in the first three proposals above.
Modem read-head sensors can be classified into two major categories, direct sensing and remote sensing. Either the media field is sensed directly by the detector element or the field variations from the medium are communicated to a remote magnetic transducer through a gapped high-permeability magnetic circuit. In both cases the transducers themselves can be of the giant magnetoresistance (GMR) or Hall-effect type, can be elements of an integrated circuit which provides signal amplification as described in U.S. Pat. Nos. 3,800,193 and 5,521,500 as well as write head elements as described in U.S. Pat. No. 5,155,643, and can be paired to provide differential detection as described in Indeck et al. However, to increase the vertical resolution of a remote sensor sensitivity must be sacrificed; therefore, the present invention refers to direct sensors which are inherently the most sensitive.
Two unshielded thin detector read-head designs have been proposed to date in U.S. Pat. No. 5,155,643 (Jones et al.) and in the pending U.S. patent application Ser. Nos. 08/435,254 and 08/688,583. Jones et al. employ a pair of rectangular metallic GMR differential transducers oriented with their planes parallel to the medium surface and separated by an insulating gap which defines the lateral (along the recording track) resolution of the assembly. But modem cost effective state-of-the-art optical lithographic techniques limit the size of the gap and therefore the lateral resolution to at best 2500 .ANG. while lateral resolutions of 500 .ANG. or more (e.g. smaller gaps) will be required for future high density recording in excess of 20 Gb/in.sup.2. The other unshielded thin detector which was proposed in the pending patent applications employs a semiconductor GMR sensor in a Corbino rectangle horizontal-plane geometry. This thin sensor (less than 300 .ANG. high) offers very high lateral resolutions of 500 .ANG.. However, owing to the physical limitations of modem materials it has a very large GMR but relatively low output voltage, even when sensing fields of the order of 500G close to the recording medium surface. The present invention proposes an alternative, high lateral resolution thin Hall-bar sensor which has at least a factor of 8 higher signal output than the Corbino sensor of the same material at H=500G. The invention also concerns an inexpensive method for commercial production of the thin sensor. This production method is based in-part on concepts previously outlined for the fabrication of horizontal-plane Corbino detectors in the pending patent applications.